The invention relates generally to the measurement of aerodynamic and hydrodynamic loads and, more particularly, to determination of unsteady aerodynamic and hydrodynamic loads in real time.
Determination of aerodynamic forces and moments on an aircraft is critical to aircraft design. Aerodynamic loads and moments predicted by theoretical models, however, generally differ from the loads and moments experienced under actual flight conditions, largely due to the dominating role of viscous effects and their interactions with the structure. To make matters worse, current aircraft aerodynamic models used in simulation and control law development are weak in the area of unsteady flow effects. The most difficult problem in developing accurate and reliable mathematical models to predict and estimate unsteady aerodynamic characteristics is associated with the inability to accurately measure and quantify viscous effects. These problems are compounded in unsteady flows by the nonlinear interactions of viscous, inertial, and aero-elastic effects.
Except for gross flight parameters like speed, accelerations, pressure, and temperature, aircraft flight characteristics are usually obtained through derived quantities, primarily from structural response to actual aerodynamic loads, which are typically measured using strain gages, accelerometers, and the like. The accuracy of these determinations may be significantly affected, however, by any time lag between the aerodynamic phenomena (the cause) and the response of the aircraft structure (the effect). Furthermore, accurate interpretation of the structural response depends on the quality of the model used to represent the structural deflections/strains as a function of the imposed loads.
There is accordingly a need for a method for determining aerodynamic and hydrodynamic loads in real time independent of any structural response to such loads.
In one aspect, the invention provides a method for determining a load on an object immersed in a fluid stream under a set of flow and attitude conditions associated with unsteady flow phenomena. The method comprises measuring surface heat transfer at a plurality of surface locations on the object under the flow and attitude conditions to provide a set of heat transfer data. The heat transfer data are used to determine an indicator surface location of at least one critical flow feature indicator. The method further comprises calculating a load coefficient using the indicator surface location of the at least one critical flow feature indicator and calculating the load from the load coefficient and the flow and attitude conditions.
In another aspect, the invention provides a load determination system for determining a load on an object immersed in a fluid stream under a set of flow and attitude conditions. The load determination system comprises a hot film sensor arrangement having at least one hot-film sensor array appliable to a surface of the object. Each of the at least one sensor array has a plurality of hot-film sensor elements. The load determination system further comprises a constant voltage anemometer arrangement having a plurality of constant voltage anemometer circuits. Each constant voltage anemometer circuit is in communication with an associated hot film sensor element and is configured to provide a signal corresponding to heat transfer from the associated hot film sensor to the fluid stream. The load determination system also comprises a data processing system in communication with the constant voltage anemometer arrangement. A signal receiving portion in the data processing system is in communication with the constant voltage anemometer arrangement for receiving signals from the constant voltage anemometers. A critical flow feature indicator extraction portion in the data processing system is in communication with the signal receiving portion and has software for processing the signals to identify the location of one or more critical flow features on the surface of the object. A load determination portion in the data processing system is in communication with the critical flow feature indicator extraction portion and has software for calculating a load magnitude for at least one load on the object using the location of one or more critical flow features.